In today's gas turbines it is an aim to burn the fuel in the combustion chamber in a lean mixture of air and fuel. Such kind of gas turbines may be called dry low emission (DLE) combustion systems, whereby the combustion of the lean fuel mixture produces low NOx rate and compact flames. “NOx” stands for mono nitrogen oxides, i.e. the chemical compounds NO or NO2. However, these systems are prone to combustion dynamics as they run in a lean regime due to the use of the lean mixture of air and fuel. Hence, combustion dynamics may arise as a result of flame excitation, aerodynamic induced excitation or insufficient damping.
The combustion dynamics may cause high acoustic noises wherein it is an aim to reduce those combustion dynamics and those noises, in particular the sound that is generated by the dry low emission combustion systems.
Therefore, in conventional gas turbines, acoustic damping of the critical frequency is performed. Thus, damping devices are installed that are placed directly to the combustion chamber or inside the casings of the gas turbines. The damping devices may be formed of Helmholtz resonator dampers or perforated liners.
Helmholtz resonators are known to be very effective at damping a critical frequency experienced by the gas turbine system. Normally the Helmholtz resonators are designed to target a single critical frequency experience at a single load point of the gas turbine. When the load of the gas turbine is altered, in particular for example between 50% and 75%, the combustion system might be prone to the combustion dynamics. The temperature due to different loads of the gas turbine may be changed and therefore the resonating frequency of the Helmholtz device might not cover the critical frequency experience by the combustion system.
In conventional gas turbines, this problem is overcome by using a set of a plurality of Helmholtz resonators with different resonating frequencies that are used to damp different frequencies generated by the combustion dynamics. For this approach, a high number of parts and costs are necessary. Moreover, the use of a plurality of Helmholtz resonators might not always be appreciable due to geometrical constraints of the gas turbine.
EP 0 111 336 A2 discloses a resonator for internal combustion engines. The resonator is adapted to absorb resonant noises from an engine by appropriately changing the length and the cross-sectional area of a tubular connecting member between the resonator and the engine. The change of the length and/or the cross-sectional area may be controlled by an actuator which is controlled by an electrical signal corresponding to a resonant frequency calculated by a computer.
WO 94/19596 A1 discloses a silencer for attenuating discharge noises in installations with pulsating gas flows. A variable Helmholtz resonator is used, wherein a regulating member influencing the Helmholtz resonator is linked to a frequency measurement device. The regulating member may be controlled by a control unit for changing the length and the cross-section of a neck of the Helmholtz resonator.
DE 196 40 980 A1 discloses a device for damping the noise of a combustion chamber. A Helmholtz resonator is used, wherein a neck part of the Helmholtz resonator provides a wall that may act as a spring-shaped wall or as a bellow that may be enlarged and reduced in its size for amending the frequency characteristics of the Helmholtz resonator.
JP 60022021 A discloses a device for lowering the noise level of an engine effectively by providing resonance chambers. The chambers are connected via a connecting pipe that comprises valves for providing an airstream.
JP 58093955 A discloses a device for reducing intake air sound and for reducing noise during engine running. Therefore, the volume of a resonance chamber may be made variable by controlling a piston for changing the volume of the resonator.
JP 60182348 A discloses a device for reducing a noise in an engine by controlling the length of a resonance passage, a sectional area and a volume in the resonance chamber. Thereby a piston is installed that may be controlled in order to change the characteristics of the resonance passage.
SU 767824 describes a bimetallic plate to which an electric current may be applied from a source via regulator. The bimetallic plates may vibrate at the inlet of a close space of a resonance.
JP 11044266 A1 discloses a resonator for enabling a compensation of frequency according to a temperature change. A communication pipe couples a gas pipe with a volume part of a Helmholtz type resonator. Communication pipe radius control means reduce the radius of the communication pipe according to a change in ambient temperature. The communication pipe radius control means comprises a leaf and an expansion and contraction member. The leaf forms an inner diameter of the communication pipe. The expansion and contraction member controls the inner diameter of the leaf according to temperature changes.